Solar-powered refrigeration holds great promise for extending the benefits of refrigeration to areas not served by reliable central station generated electricity. However, in spite of a keen interest in this prospect for over thirty years, no solar-powered refrigerator has yet achieved all the desirable objectives of simplicity, reliability, and low cost. Simplicity implies both ease of operation and also ease of manufacture, particularly in lesser developed countries. Cost is a function of cycle efficiency as well as design techniques. Since lower efficiency requires greater collector area, extreme simplicity at the expense of very low efficiency imposes a costly tradeoff.
Three fundamental initial choices are made when categorizing solar thermally-activated refrigerators. They are: whether the cycle is continuous (absorption and generation occur simultaneously or intermittent): what working fluid (i.e., refrigerant) is used (e.g., NH.sub.3, MMA, halogenated hydrocarbons, methanol, sulfur dioxide, or H.sub.2 O); and whether a solid-phase or liquid-phase absorbent is used.
This disclosure is directed toward intermittent cycle (also known as periodical) refrigerators employing preferably NH.sub.3 as working fluid and preferably liquid-phase absorbents.
Within this limited category, numerous problems are encountered in the various prior art solar-powered refrigerators. Various continuous-cycle solar refrigerators are known in the prior art, e.g., as described in U.S. Pat. Nos. 4,362,025 and 4,178,989. Those cycles have the disadvantages that a solution pump is required (e.g., percolator type, with very imprecise flow rate), and also that the evaporation and absorption steps must occur only during the very limited time (about 5 hours per day) when generation and condensation are also occurring.
Various intermittent-cycle solar refrigerators are known which employ solid-phase absorbents, e.g., as described in U.S. Pat. No. 4,586,345. When the solid absorbent is anhydrous (CaCl.sub.2, SrCl.sub.2, KSCN, or others) and the refrigerant is NH.sub.3, those refrigerators have the advantage that only dry NH.sub.3 is present, and hence aluminum construction is possible. However, they suffer the disadvantages that heat transfer with the solid particles is very poor; that shrinkage and swelling occurs leading to voids and possible ruptures; and that the heat necessary to desorb NH.sub.3 from a solid is characteristically substantially higher than that necessary to desorb NH.sub.3 from a liquid, thus requiring more solar energy for production of a given amount of NH.sub.3 refrigerant.
Various intermittent-cycle refrigerators employing either liquid or solid absorbents, but which use flame combustion as a source of heat, are known in the prior art. Included among them are the disclosures of U.S. Pat. Nos. 1,711,804, 2,446,636, 2,452,635, and 2,587,996 (all solid-phase absorbents), and also U.S. Pat. No. 2,185,330 (liquid-phase absorbent).
Numerous solar-powered liquid absorbent intermittent cycle refrigerators employing NH.sub.3 as refrigerant have been described in the technical literature over the past thirty years. Representative examples include: F. Trombe and M. Foex, "Production of Cold by Means of Solar Radiation", Solar Energy, 1, 1957, p. 51-52; R. K. Swartman, et al., "Comparison of Ammonia-Water and Ammonia-Sodium Thiocyanate as the Refrigerant-Absorbent in a Solar Refrigeration System", Solar Energy, 17, 1974, p. 123-127; A. Venkatesh and M. C. Gupta, "Experimental Investigations of an Intermittent Ammonia-Water Solar Refrigerator", National Solar Energy Convention, Report CONF-781261, December 1978, p. 675-784; and R. H. B. Exell, et al., "Design and Testing of a Solar Powered Refrigerator", Asian Institute of Technology Research Report 126, 1981, Bangkok.
One generic problem which all solar-powered, inermittent-absorption-cycle refrigerators share is how to efficiently and economically collect and retain a maximum amount of solar heat into the absorbent solution by day when it is generating, yet equally effectively cool it by night when it is absorbing. Various solutions to this problem have been tried. Due to the elevated temperature above ambient while generating, at least one layer of glazing is normally employed to admit the solar radiation to the generator yet minimize the escaping thermal radiation (heat leak). Some designs have removable glazing for nighttime cooling. Others have dampers which can be opened to admit convective air flow under the glazing at night. Designs with flat plate collectors will normally have insulation on the side away from the sun, and that can be removed at night.
Clearly none of the above techniques presents much air-cooled surface for cooling the absorption mode, typically no more surface than the solar aperture dimensions. Since air contact cooling has a very low heat transfer coefficient, this cooling technique is not efficient. Removable glazing is unwieldy in larger sizes, and mitigates against retaining a good seal against rain and dust.
Another solution preferred is to have a separate heat removal circuit built into the generator which is only activated at night when it is in the absorption mode. The evaporator end of a thermosyphon can be incorporated in the generator, and an efficient air-cooled condenser end of the thermosyphon is located at a higher elevation such that liquid returns from the condenser to the evaporator by gravity. The thermosyphon technique for removal of absorption heat is illustrated in U.S. Pat. No. 4,586,345. Note that a cutout valve is necessary in order to block liquid flow to the thermosyphon while the sun is shining, and the valve mechanism is solar-actuated. This system has several disadvantages: a completely separate air-cooled condenser plus associated refrigerant is required, which is only used at night; and when the liquid flow to the thermosyphon is cutout in order to stop the thermosyphoning action, all the liquid inventory in the thermosyphon evaporator must be boiled away before the heat removal ceases.
The five flame-actuated intermittent absorption refrigerators referenced above incorporate thermosyphons for removal of absorption heat. The U.S. Pat. Nos. 1,711,804 and 2,185,330 incoporate a separate closed-cycle thermosyphon with its own internal refrigerant, similar to the U.S. Pat. No. 4,586,345 solar patent. The other three patents, U.S. Pat. Nos. 2,446,636, 2,452,635, and 2,587,996, all directed to solid absorbents, incorporate open cycle thermosyphons which utilize the same condenser(s) and the same refrigerant as the refrigerator itself. Note they all incorporate two generator/absorber vessels, and they all locate the thermosyphon cutout valve in the liquid supply line.
Other problems found in the prior art practice of intermittent solar-powered refrigerators using high pressure refrigerants such as NH.sub.3 or monomethylamine include:
1. The choice of solar collector geometry. With flat plate collectors, providing both the necessary storage volume of refrigerant absorbent and also the good thermal contact between solar radiation and the absorbent requires both large storage vessels and many small pressure tubes welded to the storage vessel. Also, flat plate geometry presents much surface area for thermal leakage. However, the alternative to flat plate--concentrating collectors--may require tracking or frequent repositioning, which greatly increases the cost and complexity of the solar collector. This is especially true for high concentration ratios, e.g., 2.5 or more.
2. Many prior art designs incorporate the receiver directly in the evaporator, or at the same pressure as the evaporator. This requires that all of the refrigerant liquid in the receiver cools down by adiabatic flashing as the evaporator cools down. Thus much of the refrigerant is wastefully consumed, and a larger cold thermal boundary is present. When the receiver is integral with the evaporator, warm refrigerant liquid is collected in the cold box by day, contrary to the objective of keeping the cold spaces cold. Even worse is when the condenser and evaporator are physically the same component, which introduces latent as well as sensible heat to the evaporator (condenser) coil.
3. In prior art ammonia-water solar-powered refrigerators, frequently a rectifier is incorporated to reduce the H.sub.2 O content of the desorbed NH.sub.3 vapor. However, without a rectifier only about 2% H.sub.2 O accumulates in the liquid NH.sub.3 each day in a well-designed system. Furthermore, by proper design of the evaporator, and by incorporating a sensible heat exchanger between liquid NH.sub.3 to the evaporator and the fluid effluent the evaporator, it is possible to recapture refrigeration from that H.sub.2 O by subcooling the NH.sub.3.
4. In less humid climates evaporative cooling can be much better than dry air cooling, permitting wet bulb cooling temperatures on the order of 5.degree. C. below ambient temperature. Similarly, water cooling provides much better heat transfer than air cooling.
Unfortunately the prior art attempts to obtain these benefits have involved very large and costly water tanks, constructed, for example, out of porous cement, thus requiring extensive field construction. Also the porosity decreases with time, providing excessive water loss early on and insufficient wicking later on. Water availability can also be a problem.
What is needed, and included in the objectives of this invention, is a solar thermally-actuated intermittent-absorption-cycle refrigerator using NH.sub.3 as working fluid and liquid-phase material as absorbent, which:
requires only a single simple vessel for containment of the absorbent solution, and the solar radiation is directed on that vessel;
achieves a concentration ratio between about 1.5 and 2.5 but does not require tracking;
has a liquid NH.sub.3 receiver vessel separate from the NH.sub.3 evaporator and outside the cold boundary; and
has an efficient and economical means of cooling the absorbent container which does not require removing insulation or glazing, or a water tank of capacity greater than the absorbent vessel.